1. Field of the Invention
This invention relates to an ultraviolet radiation screening composition for surface impregnating polycarbonate sheet and more particularly relates to an ultraviolet radiation screening composition for on-line surface impregnating hot polycarbonate sheet with an ultraviolet radiation screener.
2. Description of Related Art
The vast majority of all organic polymeric materials undergo some mode of degradation when exposed to the high energy photons of ultraviolet radiation. The degradation manifests itself depending on the polymeric material in yellowing, discoloration, embrittlement and other loss of physical properties. Polycarbonate resin is no exception and it is, therefore, an object of this invention to provide a method of producing a polycarbonate resin article which is highly resistant to ultraviolet light degradation.
The use of ultraviolet radiation absorbers with various resins such as polyesters, polyolefins, vinyls, and polystyrene to provide protection against attack by ultraviolet radiation is known in the art. The ultraviolet radiation absorber functions by reason of its ability to screen out the damaging ultraviolet portion of light due to its very high absorptivity relative to that of the polymer. In order to qualify as a successful ultraviolet light absorber for a polymer, particularly for polycarbonate, there are several requirements which the absorber must fulfill. The absorber must have a high specific absorptivity in the range of wave lengths that are most deleterious to the polymer and that are present in the source of the exposure. The absorber must be compatible with the polymer such as polycarbonate and must not degrade the polymer with the loss of properties and increase in color. The absorber must not significantly absorb in the visible region of the spectrum or a color will be imparted to the polymer to which it has been added. The absorber must also have a sufficiently low volatility to permit its continued residence in the polymer.
Several methods are known in the prior art utilizing these ultraviolet radiation absorbers to stabilize various polymers, including polycarbonate, against ultraviolet radiation. These methods include blending the ultraviolet radiation absorbers with the polymer prior to processing, incorporating the absorbers in surface laminating or coating materials which are applied onto the surface of the processed polymer; and impregnating the absorbers in the polymer surface. The surface impregnation techniques known in the prior art include (i) using aggressive solvents to swell or soften the polymer surface thereby allowing the absorber to diffuse into the softened surface of the polymer; (ii) melting the absorber and the polymer surface in order to diffuse the molten absorber into the molten polymer surface; and (iii) partitioning of the absorber between a polymeric surface and a relatively poor solvent for the absorber held at high temperatures whereby the absorber, which is more soluble in the polymer than in the solvent, diffuses into the polymer surface.
While each of these methods can be utilized to impart improved ultraviolet stability to a polymer system, each of them has certain disadvantages. Blending the absorber with the bulk polymer results in the absorber being distributed throughout the entire polymer system. This procedure is both uneconomical, as these absorbers are usually quite expensive, and not completely successful. Since most of the absorber resides in the polymers interior instead of at the surface where it is most needed, much of the harmful ultraviolet radiation penetrates and deteriorates the surface of the polymer structure before reaching the majority of the interiorly distributed absorber. Furthermore, since the concentration of the absorber in the resin is limited by the degree of compatibility of the absorber with the polymer, using sufficiently high concentrations of absorber effective to provide surface protection generally tends to adversely affect the physical properties of the polymer. Incorporating the absorbers in surface laminating or coating materials suffers from the disadvantage of being generally difficult and expensive to use since an extra complicated processing step is required. Furthermore, difficulties are sometimes encountered in adhering the coating or laminating material to the surface of the polymer, or in maintaining continued adequate adhesion, especially after exposure to weathering. Even when the coating or laminating material adheres well, it often cannot be applied without forming unsightly streaks on the polymer surface. An additional drawback to this method is that often the physical properties of the polymer, such as impact strength, are adversely affected by these coating or laminating materials.
The surface impregnation techniques are the most desirable since the ultraviolet radiation absorbers are contained only in the surface regions of the polymer where they are needed, and ideally the surface impregnation step would be part of the on-line process for production of the sheet. Thus, for mass production of ultraviolet radiation stabilized polycarbonate sheet, it would be desirable to be able to surface impregnate the hot polycarbonate sheet as it leaves the sheet producing extrusion die. Prior surface impregnating compositions, however, do not provide an adequate processing window to permit consistant production of on-line, high quality, surface impregnated polycarbonate sheet. Such compositions are either
(i) too volatile at the elevated sheet temperatures, for example temperatures in excess of 180.degree. F., encountered immediately following sheet production downstream from the extruder thus not allowing for adequate surface impregnation thereby leaving excess oily absorber on the surface which will collect dust unless washed off in a subsequent washing step or
(ii) such compositions at elevated temperatures are so aggressive to polycarbonate that they unduly attack the surface of the sheet thereby reducing the optical quality thereof.
Thus, one object of the present invention is to provide an ultraviolet radiation screener composition for on-line surface impregnating hot polycarbonate sheet as the sheet is produced.